Group III nitride crystals usable as group III nitride substrate, method of manufacturing the same, and semiconductor device including the same

Abstract

The present invention provides a method of manufacturing Group III nitride crystals that are of high quality, are manufactured highly efficiently, and are useful and usable as a substrate that is used in semiconductor manufacturing processes. The method of manufacturing Group III nitride crystals includes: forming a first layer made of a semiconductor that is expressed by a composition formula of Al s Ga t In 1-s-t N (where 0≦s≦1, 0≦t≦1, and s+t≦1); forming a second layer by bringing the surface of the first layer into contact with a melt in an atmosphere including nitrogen, wherein the second layer includes greater defects in a crystal structure, such as a dislocation density for example, than those of the first layer, and the melt includes alkali metal and at least one Group III element selected from the group consisting of gallium, aluminum, and indium; and forming a third layer through crystal growth in the melt in an atmosphere including nitrogen, wherein the third layer is made of a semiconductor that is expressed by a composition formula of Al u Ga v In 1-u-v N (where 0≦u≦1, 0≦v≦1, and u+v≦1), and the third layer has less defects in a crystal structure, such as a dislocation density for example, than those of the second layer.

What is claimed is: 
     
       1. Group III nitride crystals, comprising:
 a first layer made of a semiconductor that is expressed by a composition formula of Al s Ga t In 1-s-t N (where 0≦s≦1, 0≦t≦1, and s+t≦1); 
 a second layer formed on the first layer; and 
 a third layer made of a semiconductor that is expressed by a composition formula of Al u Ga v In 1-u-v N (where 0≦u≦1, 0≦v≦1, and u+v≦1) and is formed on the second layer by a liquid phase epitaxy method, 
 wherein the second layer includes nitrogen and at least one element selected from the group consisting of gallium, aluminum, and indium, and 
 wherein the first layer, the second layer, and the third layer each has a crystal structure at least in a part thereof, and the defect density of the crystal structure of the second layer is higher than that of the third layer. 
 
     
     
       2. The Group III nitride crystals according to  claim 1 , wherein the defect density includes a dislocation density. 
     
     
       3. The Group III nitride crystals according to  claim 2 , wherein the dislocation density is that of at least one of edge dislocations and screw dislocations. 
     
     
       4. The Group III nitride crystals according to  claim 2 , wherein the second layer has a dislocation density that is at least 100 times higher than that of the third layer. 
     
     
       5. The Group III nitride crystals according to  claim 2 , wherein the second layer has a dislocation density of at least 10 8  cm −2 . 
     
     
       6. The Group III nitride crystals according to  claim 2 , wherein the third layer has a dislocation density of higher than zero but not higher than 10 6  cm −2 . 
     
     
       7. The Group III nitride crystals according to  claim 1 , wherein an interface between the first layer and the second layer has a concave-convex shape. 
     
     
       8. The Group III nitride crystals according to  claim 1 , wherein the second layer and the third layer are formed by immersing the first layer in a melt to carry out crystal growth in an atmosphere including nitrogen, and the melt contains nitrogen, at least one of alkali metal and alkaline-earth metal, and at least one Group III element selected from the group consisting of gallium, aluminum, and indium. 
     
     
       9. The Group III nitride crystals according to  claim 1 , wherein the third layer is formed of gallium nitride (GaN) crystals. 
     
     
       10. The Group III nitride crystals according to  claim 1 , wherein the third layer is formed of aluminum nitride (AlN) crystals. 
     
     
       11. The Group III nitride crystals according to  claim 1 , wherein the defect density includes an impurity density. 
     
     
       12. The Group III nitride crystals according to  claim 11 , wherein at least one of the first layer, the second layer, and the third layer is formed by a liquid phase method using at least one of alkali metal and alkaline-earth metal, and the impurity density is a density of impurities originating from at least one of the alkali metal and alkaline-earth metal. 
     
     
       13. The Group III nitride crystals according to  claim 11 , wherein at least one of the first layer, the second layer, and the third layer is formed using a crucible, and the impurity density is a density of impurities originating from at least one component of a material forming the crucible, the at least one component being selected from the group consisting of Y (yttrium), Sm (samarium), Zr (zirconium), W (tungsten), B (boron), Ta (tantalum), Si (silicon), C (carbon), and Ce (cerium). 
     
     
       14. The Group III nitride crystals according to  claim 11 , wherein at least one of the first layer, the second layer, and the third layer is formed using a reactor vessel, and the impurity density is a density of impurities originating from at least one component of a material forming the reactor vessel, the at least one component being selected from the group consisting of Fe (iron), Ni (nickel), Cr (chromium), Cu (copper), W (tungsten), Ta (tantalum), Y (yttrium), Sm (samarium), Zr (zirconium), B (boron), Si (silicon), C (carbon), and Ce (cerium). 
     
     
       15. The Group III nitride crystals according to  claim 11 , wherein a portion of the third layer located on an exposed surface side thereof has an impurity density of higher than zero but not higher than 1 ppm. 
     
     
       16. The Group III nitride crystals according to  claim 11 , wherein a portion of the third layer located on an exposed surface side thereof has an impurity density of higher than zero but not higher than 0.1 ppm. 
     
     
       17. The Group III nitride crystals according to  claim 11 , wherein the second layer has an impurity density of at least 100 ppm. 
     
     
       18. The Group III nitride crystals according to  claim 1 , wherein the first layer is formed on a substrate. 
     
     
       19. The Group III nitride crystals according to  claim 18 , wherein the substrate is one selected from the group consisting of a GaAs substrate whose surface is a (111) plane, a Si substrate whose surface is a (111) plane, a sapphire substrate whose surface is a (0001) plane, and a SiC substrate whose surface is a (0001) plane. 
     
     
       20. The Group III nitride crystals according to  claim 18 , wherein the substrate and the third layer can be separated from each other at any position between one side of the second layer in contact with the first layer and the other side of the second layer in contact with the third layer, and thereby the third layer alone can be used as a substrate for manufacturing a semiconductor. 
     
     
       21. The Group III nitride crystals according to  claim 1 , wherein the third layer of the Group III nitride crystals satisfies a relationship of N1>N2, where N1 denotes a defect density of its portion located on a side of the second layer while N2 indicates a defect density of its portion located on a side of its exposed surface. 
     
     
       22. The Group III nitride crystals according to  claim 21 , wherein the defect density includes an impurity density. 
     
     
       23. The Group III nitride crystals according to  claim 22 , wherein the third layer is formed by a liquid phase method using at least one of alkali metal and alkaline-earth metal, and the impurity density is a density of impurities originating from at least one of the alkali metal and alkaline-earth metal. 
     
     
       24. The Group III nitride crystals according to  claim 22 , wherein the third layer is formed using a crucible, and the impurity density is a density of impurities originating from at least one component of a material forming the crucible, the at least one component being selected from the group consisting of Y (yttrium), Sm (samarium), Zr (zirconium), W (tungsten), B (boron), Ta (tantalum), Si (silicon), C (carbon), and Ce (cerium). 
     
     
       25. The Group III nitride crystals according to  claim 22 , wherein the third layer is formed using a reactor vessel, and the impurity density is a density of impurities originating from at least one component of a material forming the reactor vessel, the at least one component being selected from the group consisting of Fe (iron), Ni (nickel), Cr (chromium), Cu (copper), W (tungsten), Ta (tantalum), Y (yttrium), Sm (samarium), Zr (zirconium), B (boron), Si (silicon), C (carbon), and Ce (cerium). 
     
     
       26. The Group III nitride crystals according to  claim 22 , wherein the portion of the third layer located on the exposed surface side thereof has an impurity density of higher than zero but not higher than 1 ppm. 
     
     
       27. The Group III nitride crystals according to  claim 22 , wherein the portion of the third layer located on the exposed surface side thereof has an impurity density of higher than zero but not higher than 0.1 ppm. 
     
     
       28. The Group III nitride crystals according to  claim 1 , wherein the third layer includes a visible-light transmission region and satisfies a relationship of K1>K2, where K1 denotes an absorption coefficient of a part of the visible-light transmission region located on a side of the second layer and K2 indicates an absorption coefficient of a part of the visible-light transmission region located on a side of an exposed surface of the third layer. 
     
     
       29. The Group III nitride crystals according to  claim 28 , wherein the absorption coefficient (K2) of the part of the visible-light transmission region located on the side of the exposed surface of the third layer is higher than zero but not higher than 100 per cm. 
     
     
       30. The Group III nitride crystals according to  claim 28 , wherein the absorption coefficient (K2) of the part of the visible-light transmission region located on the side of the exposed surface of the third layer is higher than zero but not higher than 10 per cm. 
     
     
       31. A semiconductor device, comprising a Group III nitride substrate used therein,
 wherein the Group III nitride substrate is Group III nitride crystals according to  claim 1 . 
 
     
     
       32. The semiconductor device according to  claim 31 , wherein the semiconductor device is a laser diode or a light emitting diode. 
     
     
       33. The Group III nitride crystals according to  claim 1 , wherein the second layer has a defect density that is at least 100 times higher than that of the third layer. 
     
     
       34. Group III nitride crystals, comprising:
 a seed crystal layer; and 
 a Group III nitride crystal layer formed on the seed crystal layer by a liquid phase epitaxy method using a melt, 
 wherein the seed crystal layer and the Group III nitride crystal layer are made of Group III nitride crystals expressed by a composition formula of Al s Ga t In 1-s-t N (where 0≦s≦1, 0≦t≦1, and s+t≦1), 
 the Group III nitride crystal layer is formed by bringing the seed crystal layer into contact with the melt that is in an unsaturation state and then changing the melt into a supersaturation state, and 
 in the Group III nitride crystal layer, the defect density of the Group III nitride crystals on a seed crystal side is at least 100 times higher than that of the Group III nitride crystals on an exposed surface side. 
 
     
     
       35. A method of manufacturing Group III nitride crystals according to  claim 1 , the method comprising:
 (i) forming a first layer made of a semiconductor that is expressed by a composition formula of Al s Ga t In 1-s-t N (where 0≦s≦1, 0≦t≦1, and s+t≦1); 
 (ii) forming a second layer on a surface of the first layer by bringing the surface of the first layer into contact with a melt in an atmosphere including nitrogen, and the melt including nitrogen, at least one selected from the group consisting of gallium, aluminum, and indium, and at least one of alkali metal and alkaline-earth metal; and 
 (iii) forming, on the second layer, a third layer in the melt in an atmosphere including nitrogen, the third layer being made of a semiconductor that is expressed by a composition formula of Al u Ga v In 1-u-v N (where 0≦u≦1, 0≦v≦1, and u+v≦1), and the third layer having a lower defect density defect density than that of the second layer. 
 
     
     
       36. The method of manufacturing Group III nitride crystals according to  claim 35 , wherein the defect density includes a dislocation density. 
     
     
       37. The method of manufacturing Group III nitride crystals according to  claim 36 , wherein the dislocation density is that of at least one of edge dislocations and screw dislocations. 
     
     
       38. The method of manufacturing Group III nitride crystals according to  claim 36 , wherein the second layer has a dislocation density that is at least 100 times higher than that of the third layer. 
     
     
       39. The method of manufacturing Group III nitride crystals according to  claim 36 , wherein the second layer has a dislocation density of at least 10 8    −2 . 
     
     
       40. The method of manufacturing Group III nitride crystals according to  claim 35 , wherein in the process (ii), the melt is changed from an unsaturation state into a supersaturation state, and in the process (iii), the melt is in the supersaturation state. 
     
     
       41. The method of manufacturing Group III nitride crystals according to  claim 40 , wherein in the process (ii), at least a part of the surface of the first layer is melted by the melt that is in the unsaturation state, and thereafter the second layer is formed thereon. 
     
     
       42. The method of manufacturing Group III nitride crystals according to  claim 35 , wherein a gallium nitride (GaN) crystal layer is formed as the third layer. 
     
     
       43. The method of manufacturing Group III nitride crystals according to  claim 35 , wherein an aluminum nitride (AlN) crystal layer is formed as the third layer. 
     
     
       44. The method of manufacturing Group III nitride crystals according to  claim 35 , wherein the atmospheres including nitrogen employed in the processes (ii) and (iii) each are a pressurized atmosphere. 
     
     
       45. The method of manufacturing Group III nitride crystals according to  claim 35 , wherein in the process (i), the first layer is formed on a substrate. 
     
     
       46. The method of manufacturing Group III nitride crystals according to  claim 45 , wherein the substrate is one selected from the group consisting of a GaAs substrate whose surface is a (111) plane, a Si substrate whose surface is a (111) plane, a sapphire substrate whose surface is a (0001) plane, and a SiC substrate whose surface is a (0001) plane. 
     
     
       47. The method of manufacturing Group III nitride crystals according to  claim 45 , wherein the method further comprising, after the process (iii), a process (iv) of separating the substrate and the third layer from each other at any position between one side of the second layer in contact with the first layer and the other side of the second layer in contact with the third layer. 
     
     
       48. The method of manufacturing Group III nitride crystals according to  claim 47 , wherein the process (iv) is a process of irradiating the second layer with light. 
     
     
       49. The method of manufacturing Group III nitride crystals according to  claim 35 , wherein the process (ii) is a process of forming a gallium nitride (GaN) layer as the second layer, the melt includes gallium and sodium, and the process (ii) is carried out initially at a pressure of less than a minimum pressure P (atm (P×1.013×10 5  Pa)) that is required for generating GaN crystals at a temperature (T° C.) of the melt, and then at a pressure of higher than the minimum pressure P (atm (P×1.013×10 5  Pa)). 
     
     
       50. The method of manufacturing Group III nitride crystals according to  claim 49 , wherein the temperature of the melt is in a range of 600° C. to 950° C., and the minimum pressures P (atm (P×1.013×10 5  Pa)) that is required for generating GaN crystals at various temperatures (T° C.) of the melt are as follows: 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   Mixed Melt 
                   Minimum Pressure P required 
                 
                     
                   Temperature T 
                   for generating GaN Crystals 
                 
                     
                   (° C.) 
                   (atm (P × 1.013 × 10 5  Pa)) 
                 
                     
                     
                 
                     
                 
                 
                 
                 
               
                     
                   600 
                   50 
                 
                     
                   700 
                   5 
                 
                     
                   750 
                   5 
                 
                     
                   800 
                   10 
                 
                     
                   850 
                   15 
                 
                     
                   880 
                   25 
                 
                     
                   900 
                   40 
                 
                     
                   950 
                   70. 
                 
                     
                     
                 
             
                
                
                
                
                
               
               
                
               
            
             
                
                
                
                
                
                
                
                
                
               
            
           
         
       
     
     
       51. The method of manufacturing Group III nitride crystals according to  claim 35 , wherein in the process (iii), a growth rate at which the third layer grows in a thickness direction is reduced stepwise or continuously. 
     
     
       52. The method of manufacturing Group III nitride crystals according to  claim 51 , wherein the growth rate at which the third layer grows is reduced stepwise or continuously by raising growth temperature. 
     
     
       53. The method of manufacturing Group III nitride crystals according to  claim 51 , wherein the growth rate at which the third layer grows is reduced stepwise or continuously by reducing growth pressure. 
     
     
       54. A method of manufacturing a Group III nitride substrate, comprising growing Group III nitride crystals by a method of manufacturing Group III nitride crystals according to  claim 35 . 
     
     
       55. The Group III nitride crystals according to  claim 1 , wherein the Group III nitride crystals can be used as a Group III nitride substrate.